1. Field of the Invention
The present invention relates to a method for producing a light emitting diode (LED), and particularly to a method for producing a light emitting diode including a transparent window and a transparent conductive film.
2. Description of the Related Prior Art
General processes for fabricating LEDs can be divided into up stream, middle stream and down stream. In the up stream, LPE (Liquid Phase Epitaxy), MBE (Molecular Beam Epitaxy) and MOVPE (Metal Organic Vapor Phase Epitaxy) are usually applied to producing single chips or epitaxial chips. In the middle stream, mask, dry or wet etching, vacuum vapor deposition and cutting are applied to forming dice. In the down stream, dice are placed on lead frames, and finally packaged as lamps, digit displays, dot matrix or surface mount, optionally.
In the up stream, GaAs and GaP are frequently served as substrates for depositing the light emitting material thereon. However, GaAs can absorb visible light emitted from double hetero-junction structure and GaP can absorb yellow-green wavelength, which significantly limit the application thereof. Theoretically, the external quantum efficiency (ηext) is a product of the internal quantum efficiency (ηint) and the LED emitting efficiency (ηoptical), that is, ηext=ηintηoptical. The emitting efficiency is further influenced by absorption of the material itself (ηA), Fresnel loss (ηFr), and total reflection critical angle (ηcr); that is, ηoptical=ηAηFrηcr. Among these factors, ηFr is caused by the large difference between nLED (refractive index of the LED illuminant media; ≈3.4) and nair (refractive index of the air; =1); that is, ηFr=1.84 (by ηFr=4/[2+(nair/nLED)+(ηLED/ηair)]. Therefore, if one material is provided for the LED and has a refractivity ranging between those of the air and the LED, ηFr will be greatly improved. A method for solving this problem is to coat a resin film (n=1.5), whereby the light transmission can be thus promoted 16.2%. However, the absorption of light by the substrates still exists.
R.O.C. Pat. No. 466,784 and 474,034 mentioned methods for producing the AlGaInP LED and the AlGaAs LED. In these methods, the GaAs substrate can be removed after a transparent substrate, such as glass, is attached to the LED epitaxial layer by thermal pressing or adhesives, such as spin on glass, polyimide, silicon resin, etc. Unfortunately, the adhesives easily result in bubbles at the interface. Additionally, current spreading is not satisfying by such methods, unless the cladding layer adjacent to the transparent substrate is thick enough, or a GaP layer used for oxidation barrier is thicker than 3 μm, which certainly increases the cost.
Therefore, it is desirable to provide an improved LED structure to mitigate and/or obviate the aforementioned problems.